Traditionally in commercial practice, prior to 1964, immediately after steel rod was rolled, it was coiled into bundles while still hot (either with or without air cooling in the reels). Thereafter it was cooled on a flat, chain conveyor until it was firm enough to be hung on a hook carrier without sagging, whereupon it was transferred to a hook carrier and cooled down to room temperature while hanging in the open air. The cooling took a long time and resulted in a major loss of metal (usually about 1.5%) due to the rapid oxidation of the steel at elevated temperature. In addition the metallurgical structure of the steel was poor and the rod (in the medium to high carbon content range) had to be subjected to a heat treatment (called "patenting") before it could be cold worked into a finished product. Various efforts to accelerate the cooling to reduce scale and improve the structure included (a) immersing the rod in water immediately after rolling (U.S. Pat. No. 459,903; 895,973); (b) spraying water onto the rod as it was being coiled (U.S. Pat. Nos. 854,808; 3,011,928); (c) passing the rod through a delivery pipe equipped with water spray nozzles, prior to coiling it (U.S. Pat. Nos. 1,211,277; 1,672,061); and (d) blowing air onto or through the bundles after (or during) coiling (U.S. Pat. Nos. 2,516,248; 2,810,569). By optimally combining one or moreof those techniques it was possible, on a commercial scale, to shorten the cooling time to about 30 minutes, and to reduce the metal loss by oxidation to about 1.0%. Through the years it was, of course, known that very rapid cooling and an even smaller scale loss could be achieved by increasing the application of the water, but when such was done with medium-to-high carbon content steel rod, even with very sophisticated controls (see e.g. U.S. Pat. Nos. 2,756,169 and 2,994,328), the adverse effects of surface hardening caused thereby, yielded such an unsatisfactory product that those processes were never adopted commercially for the sequential rolling and cooling of medium-to-high carbon content steel rod.
In the early 1960's when the process described in U.S. Pat. Nos. 3,231,432; 3,320,101; and 3,390,871 (which process is now commonly referred to as the "Stelmor" process) went into commercial operation, a very substantial improvement in rod quality together with a reduction in cooling time and scale loss became possible. This was accomplished by first rapidly water-cooling the rod from a rolling temperature of about 980.degree. C. to about 780.degree. C. in the delivery pipes. Thereafter the rod was formed into rings and deposited in off-set, overlapping relation on an open conveyor, and further rapidly cooled thereon by blowing air through the rings. The Stelmor process was extremely successful because it succeeded, for the first time, in providing a rod product in the medium-to-high carbon content range which was equal to an "air patented" rod. Although it did not have the quality of a lead patented rod, it still could be drawn or cold worked to a finished, saleable product in many instances without requiring any subsequent heat treatment. The savings gained by the Stelmor process were tremendous (over 10% of the price of the rod), and the Stelmor process went into immediate and widespread use.
The most difficult thing to understand about the Stelmor process, for those skilled in the art at the time, was how the rod properties could be as uniform as they were. Thus, in conventional patenting processes, it had always been necessary to take care to prevent the rod strands from touching each other because, in conventional patenting, when the strands touched each other, the reduction in cooling rate caused thereby produced soft spots in the rod (i.e. coarse lamellar pearlite--and large free ferrite deposits). On the other hand, with the Stelmor process the rod is coiled into spread-out rings on the conveyor with many parts of the rings touching in such a way that uniform cooling is impossible. In fact, the overlapped or grouped portions of the rings remain bright red in some cases as long as seven or more seconds after the individual nontouching parts of the rings turn black such that significant non-uniformity of the cooling rates from place to place along the rod is plain to see. The resulting product is, nevertheless, sufficiently uniform to meet the industry standards of a properly "air patented" rod. The explanation of this apparent nonsequitur was initially believed to be that, in the preferred practice of the Stelmor process, the air was blown more intensively onto the edges of the conveyor where there is a greater concentration of metal. In fact the earliest attempts to improve the Stelmor process involved coiling the rod in various ways to avoid accumulation of the rod at the sides of the conveyor (see e.g. U.S. Pat. Nos. 3,405,885 (Schloemann), 3,454,268 (Yawata), 3,469,429 (Schloemann), 3,469,798 (Schloemann), German Nos. 1214635, 1240541 (Demag), 1245403 (Demag) and others). Experiments, however, showed that selective blowing the air was responsible for only a minor part of the explanation, and in fact, none of the attempts to improve Stelmor by special forms of coiling and blowing have brought about anything more than minor improvement.
Eventually the reason why the Stelmor process produces acceptably uniform product was found to be due to cooling the rod rapidly after rolling so as to produce uniformly small austenite grains prior to transformation and then to cool the rod continuously and relatively rapidly through transformation. More specifically, in the Stelmor process, the rod is cooled preliminarily by water in the delivery pipes immediately after rolling. During rolling, the austenite grains in the steel are, of course, fragmented and immediately thereafter they recrystallize and start growing from extremely small size under conditions of ample excess heat above A.sub.3. Thus, they grow very rapidly and uniformly by the merger of adjacent grains. The preliminary water cooling, however, arrests the grain growth process, and, in the Stelmor process, grain sizes of about ASTM 7.5 or smaller and variations in grain sizes of less than .+-.ASTM 0.5 along the length of the rod are usual.
In conventional patenting, however, a grain size of ASTM 7.5 was normally considered undesirable for a number of reasons. First, at any given cooling rate, smaller grains will precipitate larger amounts of free ferrite due to their larger surface-area-to-mass ratio, and the precipitation of free ferrite is normally undesirable. Second, small grained products often have poorer work hardening properties due in part to their shorter free path between grain boundaries and the usual presence of more free ferrite at the grain boundaries. In the Stelmor process, however, the disadvantages expected from the small grains do not, in fact, appear in the product for reasons that are not fully understood, and, in addition, an important special benefit results from the smallness. Small grains transform more rapidly than larger grains (see Grossmann & Bain "Principles of Heat Treating" 1964, p. 71). While this has been known per se for many years, the explanation of why it is beneficial in the Stelmor process was not known. Thus, when the rod rings are cooled on the Stelmor conveyor, transformation will start first at the most exposed places where the cooling rate is highest. In fact, as the rod with high carbon content cools, one can stand alongside the conveyor and observe the redness of the most exposed parts at first diminishing until it becomes nearly black, and then, as transformation sets in, immediately turning red again due to the liberation of the latent heat of transformation. This reappearance of red color occurs first at the point where the rod has been cooled most rapidly. It then immediately spreads, however, along the rod toward the warmer places where the rod rings are closer to each other. It has been postulated (see U.S. Pat. No. 4,168,993) that this spreading causes a "triggering" of transformation along the rod, which induces transformation to proceed more rapidly elsewhere in the rod (i.e. without preliminary super-cooling). Accordingly, due both to the smallness of the grains and possibly to the "triggering" action, as soon as the transformation temperature is reached at any given place along the length of the rod, transformation starts immediately and proceeds rapidly to completion. Thus, even though the various places along the rod transform at different times, they do so at very nearly the same average temperature of transformation. This yields a product which is at least sufficiently uniform along its entire length to be equal in uniformity to a properly "air patented" rod of the same composition.
Although the Stelmor process represented a major breakthrough, there was still room for improvement. Thus, although the quality of the Stelmor rod product was an improvement over the prior art, its UTS was still about 7% to 9% below that of lead patented rod of the same grade. In addition its uniformity, although within the allowed latitude, was substantially less than that of lead patented rod. Thus, the standard deviation in UTS of Stelmor rod usually runs around 1.5 Kg/mm.sup.2, whereas the standard deviation of lead patented rod is usually below 1 Kg/mm.sup.2. In view of the fact that substantial quantitites of rod, even though processed by the Stelmor process, still require lead patenting, many attempts have been made to improve the Stelmor process to achieve the equivalent of lead patenting.
The first approach tried was to provide special forms of coiling and/or blowing in order to make the application of the air more uniform (mentioned above). Those efforts, at best, yielded insignificant improvement.
In another series of attempts to improve on Stelmor the artisans reasoned that the quality of Stelmor rod fell short of that of lead patented rod because the grain size of the prior austenite in Stelmor rod was too small. They, therefore, predicted that a much better product could be made by letting the austenite grains grow to the larger sizes (i.e. ASTM 3 to 5) used in conventional patenting (see U.S. Pat. Nos. 3,547,421 (col. 1 lines 42-75); 3,645,805, 3,783,043, and U.K. Pat. No. 1,173,037). According to those suggestions, the grain enlargement was to be done by holding the rod at high temperature for a substantial period (i.e. 12 to 30 sec) so that the grains would grow to a uniform large size (i.e. ASTM 5 or larger). Thereafter in one process (i.e. U.S. Pat. No. 3,735,966) the rod was to be cooled rapidly down to transformation temperature and then held isothermally for transformation. In the other form of this process (i.e. U.S. Pat. No. 3,783,043) the rings containing large grained austenite were to be air cooled uniformly on an open conveyor by constantly shifting the rings so as to avoid non-uniform cooling due to the overlapped places (col. 6 lines 19-24). Those processes, however, despite claims for improvement not only failed to improve on Stelmor, they were, in fact, not equal to Stelmor. The uniformly large austenite grains produced by those processes were not suitable for cooling under the non-uniform cooling conditions which cannot be avoided when rod is laid out on a conveyor, even by constantly shifting the rings.
In the wake of the failure of the attempt to improve the quality of Stelmor rod by enlarging the austenite grains, the industry then turned in the opposite direction and proposals began to appear for making the austenite grains even smaller than in Stelmor by accentuating the preliminary water cooling, and, in fact, proposals were even made to perform the entire cooling cycle with water (see U.S. Pat. Nos. 3,704,874; 4,011,110; 4,016,009; German Pat. No. 2345738, and German OS No. 2746961).
Cooling the rod entirely with water, however, is extremely difficult to control if an equivalent to at least air patented rod is to be produced. For example, the authors of German OS No. 2746961 claim that a better rod product than that of the Stelmor process can be made by immersing the rod in water directly after rolling. Those claims, however, have not been substantiated. Small samples having a good micro-structure can be made in a laboratory, but the same conditions cannot be duplicated in production. The rod can, in some cases, be drawn to as small a diameter as a normal air patented rod, but, due to non-uniformity of structure between the surface and the core of such water cooled rod, the finished product has not, so far, in most instances, been acceptable without an intermediate patenting treatment. Thus, although an advantage in terms of shortening the length of the mill can be gained by the water cooling process, the major advantages of the Stelmor process are lost, and additional complications of water recycling and control are undertaken.
In fact, although a great deal of effort has been expended over the years trying to improve the quality of medium to high carbon Stelmor rod, little, it any significant progress has been made.
In addition to trying to improve rod quality, however, a great deal of effort has also been expended by rolling mill builders over the years, in attempting to improve a number of other aspects such as increasing rod rolling speed, reducing cobbles, and also providing sufficient versatility in a Stelmor type installation to adjust it for change from Stelmor-type treatment involving rapid cooling for high carbon grades, to retarded cooling for low carbon grades, to slow cooling (in a furnace) for low alloy grades; and to provide these things at a sufficiently low cost to be economically attractive.
Since the present invention is also addressed to the solution of these further problems, in combination with improving the rod quality, the technical aspects thereof and the present state of the art relating to them should also be discussed prior to describing the invention.
The basic problem involved in simultaneously increasing rolling speed, reducing cobbles, improving rod quality, providing versatility of in-line treatments, and doing it all inexpensively is that each aspect conflicts with the other. For example, increasing rolling speed also normally increases cobbles, particularly in a Stelmor type installation. Thus, even with normal rolling speeds of today's mills, i.e. 15,000 fpm, delivery pipe cobbles are a vexatious nuisance. But yet, if one is contemplating increasing the production rate, one must also contemplate making the delivery pipes ever longer than they are today, which, in turn, increases the risk of cobbles in the delivery pipes. Of course, tonnage production rates can be increased by rolling larger rod diameters with less cobble risk, but any grains made by so doing are offset by losses downstream in the further processing of the rod. The cheapest way to reduce the cross-section of the metal is by hot rolling. Moreover, hot rolling is done without introducing work hardening into the product which often has to be removed by subsequent costly heat treatment. Thus, the economically best way is to roll the rod to the smallest diameter feasible, i.e. down to the point where the increase in the incidence of cobbles due to the smallness (i.e. weakness) of the rod commences to outweigh the advantages of small size in further processing. In view of these considerations, until the present invention, there has appeared to be but little hope of significantly increasing the production rate of hot rolled rod (that is to increase the delivery speed of no. 5 rod beyond 20,000 fpm) without at the same time escallating the cobble risk to such an extent as to negate the economic advantage of increased rolling speed.
Similar considerations apply to problems of handling the rod rings on a Stelmor type cooling conveyor and in the reforming stages in which the rod rings are projected into a reforming tub or a collector, when they reach the end of the conveyor. If the delivery rate of the rod from the rolling mill is to be raised, for instance, to 20,000 fpm for no. 5 rod, which has recently been demonstrated to be feasible, the rod will issue from the laying head at a rate of 33 rings per second, at which rate it must be carefully handled in order to avoid a serious problem both with respect to cobbling on the conveyor due to the high rate of accumulation, and in the reforming stages due to the high rate at which the rings are projected from the end of the conveyor into the reforming tub.
Rod product quality can also be adversely affected by increasing the production rate. Obviously, if the delivery rate is to be increased to achieved a rolling rate of 20,000 fpm or more, everything else must also be increased in order to achieve at least the same desired cooling conditions as are in current use for Stelmor quality rod (i.e. water cooling in the delivery pipes to 1450.degree. F. (803.degree. C.), followed by forced air cooling with at least 2" ring spacing on centers). Unless the equipment is increased proportionally the cooling conditions will be decreased from the present norm. On the other hand, a delivery pipe and conveyor of commensurate length would require increasing the length of the building by about 300' at a cost of roughly $1M for building alone ($3500 per foot), to say nothing of the extra cost of the equipment. But totally apart from these very substantial extra costs, a commensurately long delivery pipe is considered to be undesirable. This being the case, it has been assumed, prior to the present invention, that standard Stelmor quality rod (in the medium-to-high carbon content range) could not be produced if production speeds of No. 5 rod were to be increased much over 20,000 fpm, due to the difficulty of providing adequate water cooling and the cost of providing and housing a conveyor of adequate length.
In addition, providing versatility sufficient to include slow cooling, retarded cooling or even short term annealing, which is difficult enough, at the present production rates, would become proportionally more difficult if the production rates were increased. For example, one of the problems encountered in some installations for slow cooling is the stacking of the rod rings on the conveyor. If the conveyor speed is slowed down such that the spacing between rings (on centers) is less than about 1/3", the rings build up in bunches on the conveyor with the bunches periodically cascading down to the conveyor level. Projecting the rod from the laying head at a rate of 33 rings per second onto stacks of randomly varying height causes undesirable non-uniformity on the conveyor, and reforming the rod rings from the conveyor in such a state of cascading bundles is difficult, and tends to cause stoppages in production. On the other hand, if the conveyor is run at a speed at which the ring spacing is sufficient to provide for uniform laying and convenient reforming, i.e. greater than 1/3" spacing, then a very long conveyor will be needed as well as an equally long insulated chamber or furnace as the case may be if versatility is desired. For example, if a rolling speed of 20,000 fpm were to be used, a ring spacing of 1/3", and a time on the conveyor at elevated temperature only of five minutes as required for short term annealing (see U.S. Pat. No. 3,939,015), the conveyor would have to be at least 400' long to provide for the slow cooling on the conveyor plus a section on the conveyor for cooling the rod down to handling temperature after it leaves the annealing furnace.
Another problem in slow cooling, retarded cooling, and/or annealing is uniformity of treatment. One might think that placing the rings in a heavily insulated oven or in a furnace, in the form of compact, matted closely spaced rings, would provide high uniform cooling or heating conditions. Experience, however, shows that greater uniformity is still desired. Evidently, portions of the rings on the edges of the bundles simply do not cool or heat up at the same rate as other portions within the bundle.
As a result of this panorama of apparently irreconcilable conditions, the industry has been willing to settle for small grains in one or a few specific areas to the sacrifice of losses elsewhere. For example, a proposal was recently made for rolling the rod at a speed of 80 m/sec (14,880 fpm), including a delivery pipe of 142' in length and a 234' conveyor with seven forced air cooling zones. A delivery pipe of such length at such a rolling speed for No. 5 rod, however, increases the risk of cobbles. The delivery pipe can be shortened somewhat (by about 1/3) by the use of interstand cooling in the finishing mill, but even so with a 234 conveyor, at a delivery rate of 80 m/sec the ring spacing has to be so close (about 11/2") that achieving an optimum Stelmor type cooling rate for medium-to-high carbon rod is difficult. Moreover, it is also barely feasible to cool low carbon rod slowly enough for long enough on such a short conveyor at such delivery rates, together with cooling the rod when it reaches the end of the conveyor rapidly enough to permit handling in the reforming area. In fact, it has been proposed to install a water spraying station at the end of the conveyor so that the slow cooling of low carbon rod can be extended as far as possible along the conveyor. The problem, of course, with water spraying is that, in places where the rod is still at transformation temperature (in the matted-overlapped areas), the water quench will harden the rod undesirably.
One attempt to shorten the length of the conveyor which has achieved a good deal of publicity over the years (briefly touched on above) has been to drop the rings onto a conveyor into boiling water in which the steam is supposed to form a barrier which prevents chill hardening (see e.g. U.S. Pat. No. 3,788,618). It was tried in Canada in the early 1960's. Later on (around 1965) it was suggested by CRM in Belgium, and lately an English company claims to have invented it (see Metal Producing, Sept. 1979, pp. 52-53). Over the past 15 years the process has always been on "the verge" of achieving patented quality rod on a commercial scale. The most recent installation, known to applicants, has been scheduled for commercial production now for two years. Such processes, while possibly satisfactory for the production of small, laboratory controlled samples are not suitable. Thus, although the process might greatly shorten the required length of the conveyor for high carbon rod, it does not perform satisfactorily on high carbon, and it cannot be used for the slow cooling of the major tonnage item, i.e. low carbon rod.
In view of these obstacles to progress, the present state of the art discloses that the industry has literally been groping--making small gains here and there--but always pushing the limits of feasibility in one area at the sacrifice of losses elsewhere.
In fact, to date, there has been no general attack simultaneously on the objectives of increasing rolling speeds, reducing cobbles, improving quality, and providing versatility, all at low extra cost, nor has there been any apparent hope for their combined solution let alone major gains in any one area.